Cuyama Valley

Geomechanics of the Cuyama Valley Groundwater Basin

To assess the water resources of the Cuyama Valley groundwater basin, geomechanical data, along with geologic, water-quality, and hydrologic data, were collected from selected sites throughout the Cuyama Valley groundwater basin. Geomechanical data collected from the study area included using ontinuously operating global positioning system (GPS) and interferometric synthetic aperture radar (InSAR) data . The geomechanical data were used to estimate the rate of vertical land movement in the Cuyama Valley groundwater basin to determine if it is subsiding. Data from 5 GPS stations and 133 unique interferograms were analyzed. Estimates of land subsidence for the Cuyama Valley provided insight into the response of the aquifer system to groundwater withdrawal.

GPS Data

The horizontal and vertical motion of the Earth's tectonic activity in California is monitored by a continuously working network of global positioning system (GPS) stations operated by various groups, including government agencies and education consortiums. Stations within the Cuyama study area were installed as part of Southern California Integrated GPS Network that was designed to monitor plate boundary deformation and seismic hazards throughout Southern California (Hudnut and others, 2002).

Variations in the position of a GPS station can result from tectonic motion and from deformation associated with groundwater withdrawal. A study of continuous GPS data from sites in southern California determined that measured seasonal horizontal and vertical motion across a basin were consistent with simple elastic movement of the basin material responding to aquifer pumping and recharge (Bawden and others, 2001).

"GPS sites on the margin...undergo seasonal horizontal motion toward and away from the basin, while sites within the basin undergo seasonal uplift and subsidence." (Bawden and others, 2001, pg. 814).

Seasonal motion showing fluctuating compression and expansion of the aquifer sediment is a result of elastic, or reversible, deformation. This is the result of fluctuations in the pore-fluid pressure in the aquifer sediments that are less any previous maximum fluctuations, and are generally correlated with water-level changes and associated pumping. Inelastic, or irreversible, deformation occurs when the pore-fluid pressure is reduced to a value lower than the previous minimum pressure; in response, the aquifer sediments are permanently rearranged and the pore volume is reduced (Galloway and others, 1999). Generally, inelastic deformation is indicated by a multi-year trend of decline in the elevation of the land surface and does not correlate with water level recovery.

Data collected from continuously operating GPS stations indicated that the Cuyama study area is slowly moving northwest. Stations in the mountains to the west of the valley, in the hills to the east of the valley, and in the Southern Ventucopa uplands showed a net upward motion for the land surface in the region. The CUHS, in the Southern-Main zone showed an annual velocity of -7.5 mm/yr (downward), indicating significant downward motion at this location relative to the region. The cyclic variability in the daily land-surface position in the lateral and vertical directions for CUHS indicated the aquifer sediments in the area had experienced elastic deformation. However, a longer-term downward trend likely represents inelastic deformation and indicates reduced storage capacity in the aquifer sediments. In 2011, motion in all the directions increased substantially and corresponded to compression of the aquifer sediments. The cyclic variation in the position of the land surface at CUHS also correlated with water-level measurements in nearby wells, which supports the conclusion that elastic deformation was caused by groundwater withdrawals.

Land-surface position, up coordinate, in millimeters, for the GPS stations Cuyama High School (CUHS), Ventucopa Station (VCST), McPherson_CS2008

InSAR Data

Interferometric Synthetic Aperture Radar (InSAR) is an effective way to measure changes in land surface altitude. InSAR makes high-density measurements over large areas using radar signals from Earth-orbiting satellites to measure changes in land-surface altitude at high degrees of measurement resolution and spatial detail (Galloway and others, 2000).

Synthetic Aperture Radar (SAR) imagery is produced by reflecting radar signals off a target area and measuring the two-way travel time back to the satellite. The SAR interferometry technique uses two SAR images of the same area acquired at different times and "interferes" (differences) them, resulting in maps called interferograms that show line-of-sight ground-surface displacement (range change) between the two time periods. If the ground has moved away from (subsidence) or towards (uplift) the satellite between the times of the two SAR images, a slightly different portion of the wavelength is reflected back to the satellite resulting in a measurable phase shift that is proportional to displacement. The map of phase shifts, or interferogram, is depicted with a repeating color scale that shows relative displacement between the first and the second acquisitions. The direction of displacement - subsidence or uplift - is indicated by the color progression of the fringe(s) toward the center of a deforming feature.

InSAR data showed local and regional changes that appeared to be dependent, in part, on both the time span of the interferogram, seasonal variations in pumping, and tectonic uplift. Long-term InSAR time series showed a total detected subsidence rate of approximately 12 mm per year at one location, while short InSAR time series showed uplift of approximately 10 mm per year at several locations.

Persistent scatterer InSAR interferogram images for Cuyama Valley, Santa Barbara County, California, from A, May 13, 2004, to August 26, 2004; and B, August 31, 2006, to February 22, 2007.